The present invention relates to fuel assemblies to be charged into a reactor core of a nuclear power plant, particularly a boiling water type reactor, for achieving an improved safeness at a reactivity initiated accident, an improved good running performance and fuel economy advantages.
A typical example of a fuel assembly to be charged into a boiling water type reactor is shown in FIG. 12 in an elevational section. Referring to FIG. 12, a fuel assembly is generally denoted by reference numeral 1 and the fuel assembly 1 comprises a square cylindrical channel box 2 in which a plurality of fuel rods 3 containing fuel material and at least one water rod 4 acting as a neutron moderator are accommodated. These fuel rods 3 and the water rod 4 are arranged, in the form of a lattice of 8 rows and 8 columns, vertically along the axial direction of the channel box 2 and spaced with each other by axially predetermined distances by spacers 5. The fuel rods 3 and the water rod 4 are supported at their upper and lower ends by upper and lower tie plates 6 and 7, respectively.
The reactor includes a core which is constituted by a plurality of fuel assemblies 1 each of the structure described above. In the fuel assembly 1, a highly pressurized water acting as a neutron moderator and a coolant flows into the fuel assembly from the lower portion thereof and circulates therein. During the circulation, the highly pressurized water is heated and boiled, thereby generating a void. For such phenomenon, there is provided a void distribution in which a void factor is made small at the lower portion in the fuel assembly 1 and large at the upper portion thereof.
Another highly pressurized water, not boiled, flows in a gap between adjoining two fuel assemblies 1 in the core. Accordingly, there exists a difference in densities of highly pressurized waters flowing inside and outside the fuel assembly 1 in the axial and diametrical directions. However, it is known that the higher neutron moderating effect is achieved at a water area having high density and hence, at such area, thermal neutron flux causing a nuclear fussion is high and a high power is generated, resulting in non-uniform reactor power distribution. In order to obviate such phenomenon, in the boiling water reactor, fuel enrichment distribution and burnable poison concentration distribution are made different in the axial and diametrical directions in the fuel assembly 1 to obtain a flat power distribution.
In view of these facts, one example of a fuel assembly 1 provided with the fuel enrichment distribution and the burnable poison concentration distribution is shown in FIGS. 13A and 13B.
As shown in FIG. 13A, the fuel assembly 1 is composed of the fuel rods 3 arranged in a square lattice of 8 rows and 8 columns in cross section, and one water rod 4 arranged centrally in this lattice arrangement. The water rod 4 generally has a cross sectional area larger than that of the fuel rod 3. FIG. 13B shows the distribution of the enrichments of the respective fuel rods 3 and the distribution of gadolinium as burnable poison. In FIG. 13, reference numeral 1, 2, 3 and 4 are added to the respective fuel rods 3 as the different type fuel rods and these fuel rods are composed as uranium fuel rods. G represents the fuel rods containing the burnable poison including gadlinium, for example. The numerals shown in the fuel rods 3 of FIG. 13B represent the enrichments of the uranium 235 (weight %: w/o), and the numerals such as 5.0 G denotes the containing amount of the gadlinium being of 5.0 w/o.
In the example of the fuel assembly 1 of FIGS. 13A and 13B, the distribution of the enrichment of the fuel assembly 1 is divided into two portions, in which the enrichment in the axially upper area or region is made higher than that of the lower portion to make flat the power distribution of the core. That is, in FIG. 13A, the fuel rods numbered with 2, 3, and 4 are designed so as to have the enrichment distributions, whereby the entire enrichment distribution of the fuel rods 3 is made high in the upper portion of the fuel assembly 1.
Further, as shown in FIG. 13A, the fuel assembly 1 is designed so that the fuel rods 3 facing the channel box 2 and arranged near the corner portions of the channel box 2 have relatively low enrichment distributions to thereby make flat the power distribution in the diametrical direction of the fuel assembly 1.
At the arrangement of the fuel rods 3 having the enrichment distributions in the fuel assembly 1, significant weight is given to the fuel rods 3 arranged on the outer peripheral sides facing the sides of the channel box 2. This is because the fuel rods 3 arranged on the outer pheripheral sides have high neutron flux since the water passing outside the channel box 2 is not boiled.
For the reason described above, when the fuel rods 3 having the enrichment distributions are arranged at the outer peripheral portion of the channel box 2, the high power is generated by the peripherally arranged fuel rods 3 because of the high thermal neutron flux in comparison with the centrally arranged fuel rods 3. There is known a local power peaking factor as barometer of the power generation of the fuel rods 3 in the fuel assembly 1. The local power peaking factor represents a relative ratio of the power of each of the fuel rods 3 with respect to the average power of the entire fuel assembly 1 in the diametrical direction. Since the maximum value of this local power peaking factor is limited by a limited value of the maximum linear power density allowable to the fuel rod 3, there is a limit to the increase of the enrichment of the peripherally arranged fuel rods 3.
In the meantime, when the enrichment of the fuel rods arranged in the high thermal neutron flux area is increased, an infinite multiplication factor increases in comparison with a case where the enrichment is increased by the same amount as in the low thermal neutron flux area. For this reason, to relatively increase the enrichment of the peripherally arranged fuel rods as much as possible results in the increasing of the infinite multiplication factor, resulting in the fuel economy.
Therefore, it is optimally desired to design the fuel assembly so as to satisfy, as much as possible, the limit of the local power peaking factor and the increasing of the infinite multiplication factor though both are contrary to each other. In order to achieve this desire, in the fuel assembly of conventional design, the average enrichment of the fuel rods arranged at the peripheral portions facing the sides of the channel box 3 is made high at the upper area of the fuel assembly in comparison with the lower area. In such design, the peripherally arranged fuel rods provide a relatively large local power peaking factor.
The fuel assembly 1 thus designed inherently involves a problem such that the local power peaking factor in the reactor low temperature operation may further exceed in comparison with that in the reactor power operation period. This is based on the fact that, at the reactor low temperature operation period, neutron absorption effect of the burnable posion contained in the centrally arranged fuel rod G, and hence the power at the central portion of the fuel assembly, is lowered, that is, the power of the peripherally arranged fuel rods is increased. The local power peaking factor at the low temperature operation period is an important parameter in a viewpoint of retention fuel enthalpy in a reactivity initiated accident.
The reactivity initiated accident is an event which is caused, for example, at a time when a reactivity worth of a control rod is made high at a low temperature operation period such as a reactor operation starting period and such a high reactivity as may erroneously cause a prompt criticality is applied to the reactor. In such an event, the retention fuel enthalpy of the fuel rod 3 rapidly raises and hence results in the breakage of the fuel rod 3. Further, at a time when the fuel enthalpy exceeds its limit value of 230 cal/g UO.sub.2, a mechanical energy such as pressure wave is caused. Furthermore, at the low temperature operation period of the reactor, since no void is generated, the axial power distribution in the reactor is distored extremely towards the upward direction thereof. For this reason, the magnitude of the local power peaking factor in the upper area of the fuel assembly 1 in the low temperature operation period is a matter of importance in a viewpoint of the fuel enthalpy at the reactivity initiated accident.
The local power peaking factor in the low temperature operation period of the conventional fuel assembly 1 is suppressed in a range in which a high fuel enthalpy resulting in the generation of the pressure wave is not caused.
However, in the future, in order to improve the fuel economy of the nuclear power plant, there is a fear that the local power peaking factor is made further high at the low temperature operation period in view of the improvement of the burning efficiency of the nuclear fuel, which is inconvenient. However, in such a case, since it should be inhibited that the local power peaking factor exceeds the maximum value of the fuel enthalpy allowed at the reactivity initiated accident, the local power peaking factor in the low temperature operation period is itself limited. As a result, this fact constitutes a significant problem for the design of the fuel assembly, which may result in a problem of not increasing the enrichment of the fuel assembly.
In the design of the conventional fuel assembly, it is desired in the economical viewpoint to increase the enrichment of the fuel rods arranged on the peripheral side in the channel box. However, this design of the fuel assembly increases the local power peaking factor as described above, and this increasing is remakable at the low temperature operation period of the reactor. This increasing is also significantly related to the increasing of the fuel enthalpy at the generation of the reactivity initiated accident, thus being inconvenient and troublesome.